<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Document</title>
	<style>
		#box{
			width: 200px;
			height: 200px;
			background: red url() 0 0 no-repeat;
		}
	</style>
	<script>

		window.onload= function(){

			box = document.getElementById("box");

			box.style.backgroundColor = "blue"

			console.log(box.style.width)

			width =  window.getComputedStyle ? window.getComputedStyle(box,null).width : box.currentStyle.width;

			console.log(width)






		}

		// $('#box').css('width')

	</script>
</head>
<body>
	<div id="box">
		
	</div>
</body>
</html>